Magnetic disc storage devices are commonly used by host computer systems to store large amounts of digital data in a non-volatile manner. Typically, the magnetic disc is partitioned into a number of concentric data tracks, where each data track is further partitioned into a number of data sectors. To write and read data to and from a target data sector on a particular track, a recording head (read/write head) is positioned over the track by a servo controller. Then, to write data to the track, the data serves to modulate a current in a write coil of the recording head as the disc spins underneath in order to write a sequence of corresponding magnetic flux transitions onto the surface of the disc. To read this recorded data, the recording head is again positioned over the track and transduces the magnetic flux transitions into an analog read signal comprising a sequence of pulses representing the recorded data. These pulses are then detected and decoded into an estimated data sequence by a read channel and, in the absence of errors, the estimated data sequence matches the recorded data sequence.
The servo controller performs two functions: "seeking" to a selected track, and "tracking" the centerline of the track while reading or writing data to/from the track. These operations are typically carried out using one of two serving methods--"dedicated servo" or "embedded servo". Dedicated servo is typically employed in a multi-disc system where recording heads are positioned over each disc (top and bottom surfaces) by means of a single voice coil rotary actuator. One disc is reserved, or dedicated, for storing the servo data for simultaneously controlling the position of all of the recording heads. Only one recording head is active during each operation because the skew between the discs is too great to allow simultaneous tracking of two surfaces. Another drawback of dedicated servo systems is an inherent constraint on the track density (number of tracks per inch or TPI). The skew between the dedicated servo surface and the other data surfaces disrupts the tracking operation if the track density is too high.
Embedded servo overcomes the TPI limitation of dedicated servo by recording servo data (in the form of intermittent spokes or wedges) embedded between the data sectors on each surface of the disc. In this manner, the recording heads can be positioned independently, which allows accurate tracking even at very high track densities. However, in a multi-surface embedded servo system where the recording heads are positioned by a single VCM rotary actuator, only one recording head can be active during each read or write operation. Again, this is due to the skew between the disc surfaces, and it prevents increasing the throughput of the recording system because it prevents accessing the disc through multiple recording heads simultaneously. As the speed of microprocessors increases and data intense applications proliferate (e.g., audio/video applications), the data transfer rate of storage devices will represent a significant bottleneck.
There are prior art recording devices that address this concern by providing simultaneous access to the disc through multiple recording heads, where each head is controlled independently using separate VCM rotary actuators. On such system is disclosed in U.S. Pat. No. 5,355,486 entitled "SYSTEM FOR ALLOCATING TASKS BETWEEN TWO ACTUATORS SERVICING THE SAME MAGNETIC DISK MEDIA IN A SINGLE DISK DRIVE." Obviously, the problem with this design is the increased cost associated with the additional hardware for each VCM rotary actuator, as well as the increased cost associated with adding servo control electronics to control the motion of each actuator. Additionally, it requires separate read channels for detecting and decoding the digital data from the analog read signals emanating from each recording head. Due to the skew between disc surfaces, the read channels must operate independently and would likely reside in separate integrated circuits to avoid cross talk and noise as a result of independently clocking each circuit.
There is, therefore, a need for a cost-effective way to simultaneously access multiple recording heads positioned over a disc storage medium in order to economically increase the throughput. It requires efficient use of actuator hardware, as well as cost effective designs for the servo control electronics. Furthermore, it requires efficient data formats and buffering in order to take full advantage of disc access through multiple data streams. Still further, it requires cost effective read channel circuitry that preferably resides on the same piece of silicon within a single integrated chip.